The field of this invention is the upgrading of phosphoric acid made by the `wet` process. Such phosphoric acid contains fluorine in amounts sufficient to render the acid unacceptable for many uses such as animal feed additives, for use in foods for human consumption or as a raw material for the preparation of phosphate compounds used in detergent formulations. Also, the presence of fluorine in `wet` process acid causes post-precipitation of complex fluorides which hamper production of `merchant` acid grades of phosphoric acid. For some end uses, the presence of fluorides in the acid is not detrimental but it is likewise not beneficial and may become a source of fluoride pollution. It has therefore been recognized that valuable fluorides may be obtained from the acid and their value exceeds the cost of extraction by many fold.
Many different methods have been used to remove fluorine from phosphoric acid. The use of salts to precipitate insoluble fluorine containing compounds from the acid has not been overly successful since most precipitating agents also precipitate substantial quantities of phosphoric anhydride and such losses cannot be born economically.
Fluorine containing phosphoric acid has been subjected to vacuum concentration which removes some of the contained fluorine as a mixture of hydrogen fluoride and silicon tetrafluoride. Such methods create difficult recovery problems, give rise to waste disposal and pollution problems and do not remove sufficient fluorine to greatly enhance the quality of the concentrated phosphoric acid. The commercial practice of this vacuum process on acid containing 160 pounds of fluorine per ton of phosphoric anhydride will remove less than 100 pounds of fluorine leaving about 70 pounds in the acid.
An improved recovery system also utilizes vacuum concentration of the acid to generate fluorine-containing vapors from the impure acid. These vapors are a mixture of hydrogen fluoride and silicon tetrafluoride and are reacted to form fluosilicic acid (H.sub.2 SiF.sub.6). While such a process may be operated to generate about 65% of the fluorine contained in the feed phosphoric acid, recoveries of fluorine from the vapors are less than 85% and, even at such recovery levels, the concentration of the product fluosilicic acid is too low to permit its sale on the open market without further processing. It is possible to so operate the recovery system to produce an acid of concentration sufficient to meet market requirements but this further lowers fluorine recovery to levels deemed too low to meet reasonable economic or pollution criteria.
A further improvement in a fluorine removal process comprises the addition of silica, such as diatomaceous earth, to concentrated impure acid (54% P.sub.2 O.sub.5) which results in improved fluorine removal efficiency. Such a process has required a temperature of 250.degree. F at ambient pressures and water addition to maintain phosphoric F. concentration at a constant and high level.